Adiletta U.S. Pat. No. 3,053,762, patented Sept. 11, 1962, provides a filter material having a substrate of woven, inert cloth, such as glass, upon which other fibers, such as glass, are deposited in a controlled blend and quantity and locked firmly to the substrate. The resulting sheet material is impregnated with a thermoplastic material such as polytetrafluoroethylene, polytrifluorochloroethylene or a silicone-type resin to impart special characteristics to the material. The material is then dried and cured or fused to fix the impregnant thereon. Several types and thicknesses of woven cloth may be employed as the substrate, and the pore sizes of the material may be predetermined and controlled by the type and amount of glass fibers applied thereto.
Polytetrafluoroethylene and, to a lesser extent, polytrifluorochloroethylene render the substrate hydrophobic, and impart high strength and inertness, as well as resistance to deterioration at elevated temperatures, because of the high softening point of these polymers. Silicone-type resins also impart hydrophobicity to the substrate, and resistance to deterioration at elevated temperatures, after curing of the polymer, but do not impart high strength.